1. Technical Field
The present invention relates to a crankshaft assembly of an internal combustion engine, and particularly to a crankshaft assembly of a compact V-type engine.
2. Background Art
The crankshaft assembly of an internal combustion engine must possess dynamic balance as well as static balance in order to restrict undesired vibration. More specifically, the sums of inertia forces and inertia couples of the crankshaft assembly have to be adjusted to substantially zero.
Referring to FIG. 3 of the accompanying drawings, a crankshaft 1 of a six-cylinder V-type engine comprises crankpins 2, 3, 4, 5, 6 and 7 spaced apart from each other by predetermined angles in the circumferential direction of the crankshaft 1, crankjournals 8, 9, 10 and 11 respectively supported by bearing members (not shown) of a crankcase (not shown), and crankarms 12, 13, 14, 15, 16, 17, 51, 52 and 53 for connecting the crankpins with the crankjournals. The crankshaft changes the reciprocation of a piston (not shown) to rotation. The crankarms 12-17 and 51-53 are respectively provided with balance weights in a manner such that both total static balance (inertia forces) and dynamic balance (inertia couples) of the crankarms become zero. The crankshaft can rotate smoothly or with less vibration and noise due to the balance weights. The balance adjustment of the crankshaft assembly during assembly generally depends on dummy weights mounted on the crankpins 2-7.
The crankshaft is also provided with a pulley 19 at one end 18 thereof (front end) and a flywheel 21 at the other end 20 thereof (rear end). The pulley 19 drives, for example, a generator and an air conditioner compressor mounted in the engine via a belt. The flywheel helps maintain smooth continuous rotation of the crankshaft and transmits driving power.
In the engine having the crankshaft of FIG. 3, a couple is produced due to an arrangement of the crankpins 2-7 when the crankshaft 1 rotates. Specifically, the resultant force of centrifugal forces (inertia) resulting from reciprocating members, such as pistons, piston rings, and gyrating masses which are considered attached to the crankpins 2-7 becomes zero, as viewed in the axial direction of the crankshaft, as long as the crankpins are equal to each other in weight. In this case, however, a clockwise moment (inertia couple) appears around the center point 0 of the crankshaft. Therefore, the inertia couple should be eliminated when the crankshaft 1 rotates within the speed of ordinary operational range, in order to prevent noise and vibration.
Conventionally, the crankshaft 1 is provided with balance weights 23 at positions opposite the dummy weights 22 so as to balance with the dummy weights, as schematically shown in FIG. 4, in order to eliminate the inertia couple. In such an adjustment, in most cases, the crankarms 12 and 17, which are the furthest crankarms from the center 0, are made thicker (thicker portions serve as the balance weights) rather than attaching real bodies. The crankarms 12 and 17 are chosen since a larger moment is produced in the case where the balance weights are mounted on the furthest crankarms than the case where the weights are the balance weights and are mounted on the crankarms 13 and 16.
Although compact engines are well known in the art, there is a demand for even smaller engines. In designing an engine generally, the balance weights required for the crankshaft are determined by the piston, connecting rods and other fundamental elements of the engine. Therefore, the engine has to be designed larger in height and width in order to reduce the length thereof. In this case, the distance from the center axis 40 to the balance weight becomes large. An engine smaller in height and width (thicker balance weights) can be designed if a longer dimension is allowed in the longitudinal direction of the engine. In other words, something has to be sacrificed, and it is difficult to design an engine more compact than conventional compact engines by adjusting the balance of the crankshaft itself.
Another well-known way of balancing with the dummy weights 22 is schematically illustrated in FIG. 5. In this case, the balance of each element, such as crankshaft 1 and the flywheel 21, is ignored. Importance is placed on the balance of the combination of the crankshaft 1, pulley 19, the flywheel 21, and the dummy weights 22. Therefore, the pulley 19 and the flywheel 21, which have been separately manufactured as balanced members, are provided with imbalances (24) equivalent to the balance weights of the crankshaft such that the combination of the elements is made balanced. In this arrangement, the length of the crankshaft does not elongate, and the weight of the same does not increase. However, the damper pulley 19 and the flywheel 21 are provided with the imbalances prior to assembly. This means that a particular set of the pulley and flywheel is only usable with a corresponding particular crankshaft and engine. Therefore, if there is mismatching, the combined elements cannot function properly in the product. Also, an exchange of he flywheel and damper pulley is generally impossible.